The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Today, many operators are using simple water frac treatments to stimulate production from tight sand and shale formations. Water is preferred because it is cheaper than polymer-based treatments. The volumes of water used in fracturing treatments range from tens of thousands gallons to more than 5 million gallons. Water used in fracturing process is obtained from variety of sources which include freshwater supply wells, chlorinated city water, rain water, pond water, lake water and occasionally frac flow-back water. Each of these water sources will have some level of bacterial contamination.
Since large volumes of water are pumped during fracturing treatments resulting in near wellbore cooling, this provides a favorable temperature for bacteria growth. Although the frac water is treated with biocide, post-fracture reservoir souring, the production of hydrogen sulfide by sulfate reducing bacteria (SRB) has recently been reported.
Biocides are very effective in controlling the SRB in surface facilities and flowlines, but 100% kill rate in reservoir is difficult to achieve. SRB bacteria can be planktonic (free floating) and sessile (attached to surfaces). Given the exponential growth of sessile SRB, only one bacterium needs to survive the biocide innoculation to establish large colonies in the reservoir. Once formed into colonies, sessile SRB protect themselves with a biofilm that will inhibit the ability of the biocide to act on the colony.
The presence of sulfides (e.g., H2S, HS−, and S2−) in fluids poses serious problems due to their toxicity and corrosive nature. It is well known that the presence of sulfides in many fluids is a consequence of the reduction of sulfates to sulfides by SRB. SRB are routinely found in water associated with oil production systems.
The requirements for SRB activity and growth include an anaerobic environment containing adequate nutrients, such as an electronic donor. A typical electron acceptor is sulfate, which produces hydrogen sulfide upon reduction. Typical electron donors include volatile fatty acid and hydrocarbon. The volatile fatty acids are naturally occurring compounds that include acetate, propionate and butyrate in formation waters. These fatty acids are the key to the growth of SRB in the reservoir. Previously the sulfate was considered the prime cause of sulfide generation. Certainly the presence of sulfide is necessary but the volatile fatty acids can provide essential and necessary carbon source for SRB activity and resultant sulfide generation. It should be noted that available carbon source (i.e., volatile fatty acids) in formation waters is limited. By providing nitrate as an alternate electron receptor to replace sulfate, a viable treatment strategy could be based on microbial nitrate reduction which provides approximately three times more energy than the reduction of sulfate. Sulfate reduction requires eight electrons per mole while nitrate reduction utilizes only two electrons per mole. This promotes nitrates to be the preferred electron acceptors when both sulfate and nitrate are present. Consequently, bacteria that utilize nitrate will dominate the bacteria population by competing directly for the limited carbon (volatile fatty acids) energy source in the formation waters. This leads to the exponential increase of nitrate reducing (NRB) bacteria population that out-competes the SRB population for the limited food sources content. Besides competition, the formation of nitrite, which is formed as an intermediate product of the NRB metabolic activities, affects the indigenous bacteria population. Nitrate is inhibitory to some SRB species and acts as a chemical scavenger which contributes to sulfide removal.
Some prior arts intended to use said teachings. Especially, U.S. Pat. No. 5,405,531 discloses the use of nitrite, nitrate and/or molybdate to control the production of hydrogen sulfide by sulfate reducing bacteria in aqueous systems. In the same way, U.S. Patent Application No. 2005/0238729 promotes the use of glutaraldehyde and nitrite to synergistically inhibit sulfide production by sulfate reducing bacteria. In the same field, U.S. Patent Application No. 2008/0032949 discloses the use of embedded biocide made of Tetrakis(Hydroxymethyl) Phosphonium Sulphate (THPS). The liquid-based THPS reacts or interfere with the performance of commonly use oxygen scavengers.
None of the prior arts directly refer to method of controlling the post-fracture reservoir souring. And further, it is apparent that efficacy of SRB mitigation controls needs to be improved if the post-fracturing reservoir souring is to be effectively controlled. There is thus a need in the art for a cost-effective, efficient and easy treatment of the post fracturing water.